Diel variation in density, length composition, and feeding activity of juvenile alewife, Alosa pseudoharengus Wilson, and blueback herring, A. aestivalis Mitchill, at near-surface depths in a hydroelectric dam impoundment

Catches ofjuvenile blueback herring in near-surface (0 to 1.5 m depth) waters, although greater at night than during the day, did not exhibit a strict negative phototropism because catches increased significantly with increasing illumination level during the forenoon and decreased with decreasing illumination during the afternoon. Juvenile alewife were absent from near-surface waters during daylight. Feeding activity by blueback herring increased between dawn and dusk and ceased or slowed overnight but was not strictly positively phototropic. Low power (1-β) of statistical tests due to high sampling variability and low (in hindsight) sample sizes precluded conclusions about the absence of die1 and spatial differences in the pattern of density, length composition, and of feeding activity of juvenile Alosa in those cases where the null hypothesis could not be rejected. Acceptable statistical power would require substantially increased (perhaps impractically so) sample sizes aided by an increase in a (probability of a type I error) from 0.05 to 0.1.